1. Field of the Invention
The invention relates to a middle electric motor drive unit, and more particularly to a middle electric motor drive unit for an electric bicycle.
2. Description of the Related Art
Electric bicycles are widely used nowadays. Conventional electric bicycles are divided into five types: rear-wheel driven bicycles, front-wheel driven bicycles, front-and-rear driven bicycles, center-shaft driven bicycles and pedal-shaft driven bicycles.
In China, most electric bicycles are rear-wheel driven bicycles that use hub external rotor motors.
Compared with rear-wheel driven bicycles using hub external rotor motors, advantages of the center-shaft driven bicycles comprise: firstly, structure of the bicycle, particularly weight distribution thereof, is reasonable, since drive parts are installed at the center of the bicycle, and thus center of gravity thereof is very low and stability during riding is good; secondly, since no external force from a cyclist and goods carried thereby is applied to a motor shell, hidden dangers, such as breakdown of the housing or a wheel spoked disc or wire, increased noise, and so on, caused by deformation of a housing of the motor, an uneven air gap between a rotor and a stator of the motor, and insufficient material strength of materials are not to occur; thirdly, front and rear shafts/wheels can employ normal front and rear shafts/wheels, which makes disassembly and replacement thereof simple and convenient; fourthly, since an external/internal gear transmission of bicycle can be directly installed, and a mechanical speed variator is disposed in the rear shaft drive, which makes the electric bicycle convenient for use and adjustment, and further improves riding performance under bad conditions such as climbing, head wind, overloading, and so on; fifthly, design of the center-shaft driven bicycle is flexible, and not limited by axial or radial dimension of the front wheel and the rear wheel, which makes the bicycle simple and artistic; sixthly, the center-shaft drive features better tightness and reliability; seventhly, since less metal material is used, the bicycle and the motor are light, which reduces no-load current and light-load current, improves power performance of the electric bicycle, and meets requirements for a light-weight, power-assisted, upscale, and high-efficient electric bicycle.
However, there are several problems with the center-shaft driven bicycle: firstly, since weight and riding force of a rider are directly applied to a housing of a middle motor drive, strength of materials and structure and reliability of the bicycle are poor, and the bicycle features high failure rate; secondly, the bicycle features too large volume, complex structure, high production difficulty and cost, large noise caused by multiple reduction, and low transmission efficiency; thirdly, since a center-to-center distance between the pedal shaft and an center output shaft of the drive is fixed, the minimum wheel diameter that can be selected and used by the bicycle is limited and cannot be changed, which limits application range and flexibility of the middle motor drive, and further affects application and operation comfort of the electric bicycle requiring wide range and diversity of wheel diameter; fourthly, a center-axis torque sensor has a one-way clutch, which features complex structure, and high production difficulty and cost, and has special size, and a normal and cheap flywheel can not be used; fifthly, even if a core sensor based on multi-shaft acceleration measurement is used to replace the pedal-axis torque sensor, it still has problems such as large size, small measurement range, and poor linearity; sixthly, although some bicycles employ motors separately disposed with the pedal shafts, and a single chain transmission technique is used, however, since a flywheel on an output shaft of a motor is engaged with a lower edge at the top of a chain, which reduces the number of teeth of the flywheel engaged with the chain, and work life of the flywheel and the chain, and causes severe abrasion of chain teeth, and small contact angle, in addition, since the flywheel needs to transmit the chain to a chain wheel during operation, if a one-way clutch on the chain wheel is inflexible and chain cannot be transported to the large sprocket, the chain is easily detached, which causes failure of the electric bicycle; moreover, the motor occupies a position of getting on or off the bicycle for a lady, hence the motor is not suitable for a lady's bicycle.